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Try breeding season how to play PMC Labs and tell us what you think. Learn More. Reproductive success usually declines in the course of the season, which may be a direct effect of breeding time, an effect of quality individuals with high phenotypic or environmental quality breeding earlyor a combination of the two. Being able to distinguish between these possibilities is crucial when trying to understand individual variation in annual routines, for instance when to breed, moult and migrate.
We find that the qualitative were independent of manipulation technique inducing replacement clutches versus cross-fostering early and late clutches. Given that the two techniques differ strongly in demands made on the birds, this suggests that potential experimental biases are limited. Overall, the evidence indicated that date and quality are both important, depending on fitness component and species, although evidence for the date hypothesis was found more frequently.
We expected both effects to be prevalent, since only if date per se breeding season how to play important, does an incentive exist for high-quality birds to breed early. We discuss mechanisms mediating the seasonal decline in reproductive success, and distinguish between effects of absolute date and relative date, for instance timing relative to seasonal environmental fluctuations or conspecifics.
The latter is important at least in some cases, suggesting that the optimal breeding time may be frequency dependent, but this has been little studied. A recurring pattern among cross-fostering studies was that delay experiments provided evidence for the quality hypothesis, while advance experiments provided evidence for the date hypothesis.
This indicates that late pairs are constrained from producing a clutch earlier in the season, presumably by the fitness costs this would entail. This provides us with a paradox: evidence for the date hypothesis le us to conclude that quality is important for the ability to breed early. Of these two decisions, selection pressures on clutch size have a long history of being studied experimentally, and the consequences for parents and offspring of rearing different s of young are relatively well known Dijkstra et al.
However, decisions regarding laying date and clutch size are closely related, since the of young that can be reared with a given effort depends on the timing of reproduction when, for example, food availability varies seasonally, and indeed clutch size typically varies with laying date Klomp This raises the question as to what extent clutch size variation can be understood independently of an understanding of the effect of reproductive timing.
The fitness consequences of variation in timing of reproduction have been described extensively since the seminal paper by Perrinsand on the descriptive level as opposed to the experimental levelwe therefore have good knowledge of seasonal patterns in reproductive success. However, the causes of these seasonal patterns are not well understood, since the effects of the actual timing of breeding are confounded with quality. Thus, the seasonal pattern in reproductive success may either be a consequence of timing per se the date hypothesisaffecting all individuals in the same way, or reflect quality differences between breeders, irrespective of the timing of breeding the quality hypothesis.
For example, early-breeding females may breed on territories that are rich in food Daan et al. This is unfortunate, because when considering optimal breeding time we are primarily interested in the ultimate consequences of altering breeding time for the average individual Baker ; Cuthill Thus, there is a need for experiments to disentangle the contributions of variation in time and quality to natural seasonal trends in reproductive success and other fitness components, to thereby obtain an unbiased estimate of the fitness consequences of breeding time i.
With a few pioneering exceptions, the experimental analysis of the fitness consequences of laying date started relatively recently, in the s. In this paper, we aim to provide an overview of the experimental analyses of the fitness consequences of variation in time of breeding using birds as a model system. More specifically, we address the question as to what extent seasonal variation in fitness components can be attributed to effects of time, quality or a combination of the two.
Our perspective in this paper is explicitly phenotypic, in the sense that the timing experiments we describe aim to estimate the fitness consequences of phenotypic variation in breeding time for the mean genotype. Deing the perfect experiment, in which only the focal trait is manipulated, is difficult for most traits, but seemingly impossible for manipulations of breeding time. The problem is that we have no direct control over the timing of reproduction, and consequently can only manipulate timing indirectly.
This introduces the problem that timing manipulations potentially also have effects on other traits such as conditionand the effects attributed to manipulated timing may equally be attributed to other traits that were inadvertently manipulated. We therefore start with an overview of the experimental techniques to manipulate breeding time that are known to us, and the advantages and disadvantages associated with the different methods table 1.
This is followed by an overview of the table 2where we try to draw general conclusions regarding the importance of time and quality in causing seasonal variation in reproductive success and other fitness components. Figure 1 provides a schematic overview of the possible outcomes of timing manipulations, and what can be concluded regarding the importance of timing and quality for different outcomes. Schematic of timing experiments delay only and the interpretation of .
Fitness components can be compared between the EC group EC; open dot and the delayed group filled dotsyielding information on the effect of timing. The delayed group can further be compared with pairs breeding at the same time natural seasonal trend, or LCsyielding information on the effect of quality. When the delayed group has a performance that is lower than the ECs, this provides evidence for a direct timing effect. When the delayed group has a performance that is higher than the performance of birds breeding late naturally, this provides evidence for an effect of quality.
Note that the performance of the delayed group may be intermediate to the two possible outcomes shown, providing evidence for a mixture of effects of time and quality. Techniques to manipulate time of breeding. of studies that manipulated time of breeding through inducement of replacement clutches R or cross-fostering clutches differing in hatch date CF. Indicated are also cases when synchrony or individual optimization best explained the .
In many species, removal of a clutch induces the pairs to produce a replacement clutch, thereby creating an artificially delayed group of breeding birds. The reproductive decisions such as clutch size and success of these pairs can be compared with a late control group LC composed of pairs with a natural late hatching date matching the actual hatching date of experimental pairs figure 1. A variant of this technique is to remove a clutch to induce a replacement clutch as above but fostering the removed clutch to a pair with identical early expected hatching date, letting that pair rear the removed clutch and use it as an early control EC; De Neve et al.
This procedure has the advantage of allowing a paired comparison between early- and late-hatched broods with on average the same genetic background, but has the disadvantage that delayed pairs rear their own young while ECs rear foster young. A general advantage of inducing replacement clutches is that birds have the opportunity to adjust, for example, nest characteristics, clutch size and egg traits to the new time of breeding, allowing an experimental analysis of the effect of time of breeding on these traits. There are two obvious disadvantages of this method. Secondly, this technique allows one to delay breeding, but yields no information on the fitness consequences of breeding earlier.
One risk associated with this manipulation is that the pairs that produce a replacement clutch are a non-random sample of the population, in the sense that pairs that produce a replacement clutch may differ in quality from those that do not. Such an effect could occur in either direction.
Birds of high quality may be more likely to produce a replacement clutch because they are better able to cope with the added costs or can expect larger benefits. Alternatively, birds of high quality may be maximizing fitness by not producing a replacement clutch when refraining from breeding increases the likelihood that they survive to breed another year while low-quality breeders may have low survival probability even when they refrain from breeding.
Whatever the direction of the effect, it would bias the comparison between the delayed birds and both the EC group and the naturally late-breeding birds. There is no real solution to this problem, but one can reasonably assume that such biases will be limited when a high proportion of experimental pairs start a replacement clutch. When a non-negligible proportion of pairs refrain from starting a replacement clutch, one can estimate to what extent selection on phenotypic quality occurred, comparing e.
This technique consists of cross-fostering clutches that differ in their expected hatching date, thereby manipulating hatching date and consequently the time when pairs have young in their nest. As with the induction of replacement clutches, reproductive success and other fitness components can then, for example, be compared between delayed pairs and pairs breeding late breeding season how to play LCand also with the EC pairs figure 1. An advantage of this method is that the hatching date can be manipulated both forward and backward in time.
The main disadvantage of this method is that timing is manipulated through a manipulation of incubation effort. When incubation is costly in terms of fitness de Heij et al. If however the seasonal trend in reproductive success is not linear, e. Brinkhof et al. Further disadvantages of this method are that the scope for manipulation can be small when breeding time is highly synchronized and, linked to this, that manipulations are only possible within the naturally occurring reproductive window. Both disadvantages can, in principle, be overcome by cross-fostering clutches with other populations with earlier or later breeding seasons, but such experiments, to our knowledge, have not yet been carried out.
One aspect of this manipulation that can be either an advantage or a disadvantage, depending on the details of the question asked, is that a natural shift in breeding time would often be accompanied by a change in clutch size, while normally clutches with similar of eggs are exchanged but see Wiggins et al. This can introduce a bias, in particular, when there is a steep seasonal decline in clutch size, because it will entail that for practical reasons, relatively small clutches for the time of year will be delayed, while relatively large clutches will be advanced.
Note however that this can be resolved by combining eggs from different clutches. Another disadvantage is that hatching date is shifted from the perspective of the parents, but not from the perspective of the clutch. If seasonal variation in reproductive success is caused in part by seasonal variation in egg characteristics, one could erroneously conclude that there is evidence for a causal effect of time of breeding, while it is in fact due to egg characteristics whose association with breeding date has not been broken by the experiment.
Such a scenario is not unlikely, because evidence is increasing that there are long-lasting effects of egg composition on offspring performance Schwabl ; Heeb et al. It is well established that supplementary feeding advances the start of reproduction in many species Daan et al. Thus, in this way, birds can be tricked into breeding at a time when they would not normally have started reproduction, and if feeding is stopped after the start of laying or soon thereafter, this can be interpreted as a timing manipulation Nilsson An important advantage of this method is that breeding time can be shifted to before the time that birds start breeding naturally.
A disadvantage is that there may be carry-over effects of the extra food on the condition of the parents, in the sense that reproductive performance of the experimental birds may be higher as a consequence of the supplementary food, even when they were only fed up to the start of laying. A further disadvantage is that birds may have adjusted not only the laying date to the over estimated food availability, but also other traits such as the quality and breeding season how to play eggs Nilsson Consequently, the reproductive performance that is measured may not be an unbiased estimate of the reproductive performance achieved, had the birds started laying earlier without the misleading information.
It is well known that it is possible to temporarily arrest the development of avian embryos by storing freshly laid eggs at low temperature. When the pair is given substitute eggs to ensure the clutch is not deserted, the original eggs can be returned to the nest at a later date to be incubated to hatching, creating an experimental delay of the hatching date.
In many ways, the advantages and disadvantages of this manipulation are similar to those of the cross-foster manipulations and we have categorized them as such in table 2except that this manipulation yields delayed hatching dates, but no advanced hatching dates.
An advantage of this manipulation is that eggs hatch in the environment they were potentially adapted to with respect to egg traits, such as concentrations of immunoglobulins and hormones, insofar as the adjustments made were independent of temporal variation on the time scale of the manipulation. A further advantage of this manipulation, in particular breeding season how to play populations in which breeding is highly synchronized, is that the hatching date can be delayed for a longer period than would have been possible with cross-fostering early and late clutches. One disadvantage of this manipulation is that temporary storage at low temperature may reduce hatching success Wiggins et al.
Timing of arrival can be manipulated by temporarily holding birds and releasing them at a later date. This approach can be applied, for example, to migratory birds, manipulating time of arrival on the breeding grounds Cristoland to fledglings, manipulating the time when they winter flocks Nilsson A disadvantage of this method is that holding wild-caught birds in captivity is not always straightforward, making it difficult to rule out adverse effects on the phenotypic quality of the birds.
One study that avoided this problem bred captive kestrels throughout the year through photoperiod manipulations, and released the fledglings at a fixed age, thereby obtaining information on their survival prospects throughout the year Serge Daanpersonal communication.Breeding season how to play
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The timing of birds' breeding seasons: a review of experiments that manipulated timing of breeding